Robust titanium suboxide anodes doped by sintering enhance PFOS degradation in water

Per- and Polyfluoroalkyl Substances (PFAS) are a class of persistent organic pollutants that are ubiquitous in the environment, while PFOS is one most representative PFAS of extraordinary persistence. Electrochemical oxidation (EO) is promising for destructive treatment of PFAS in water, and Magnéli phase titanium suboxide (TSO) is regarded as one of only few suitable anode materials for this application. We herein conducted an in-silico survey with Density Functional Theory (DFT) simulations to identify possible beneficial dopant elements, and then prepared TSO anodes doped with Niobium (Nb-TSO) or Cerium (Ce-TSO) by sintering. The doped TSO thus prepared exhibited great robustness, having service lifetimes longer than the pristine Ti4O7 anode, making them useful for EO applications in PFAS treatment. PFOS degradation by EO using Nb-TSO anode was faster than that on the pristine Ti4O7 anode, with energy consumption approximately 1.8 times lower. Further characterizations and DFT simulations reveal that the enhanced efficiency of Nb-TSO anode is attributed to its reduced charge transfer resistance and increased effective electroactive surface area (EESA). The EESA of the Ce-TSO anode was reduced in comparison to the pristine Ti4O7, but PFOS degradation rates normalized by EESA were increased significantly for EO with Ce-TSO anode, due to its increased oxygen evolution potential (OEP) and hydroxyl radical production. The doped TSO anodes prepared in this study by sintering will be useful in EO treatment of PFAS-contaminated waters, with improved service life and performance, and the study provides understandings to guide further improvements of the TSO anodes via doping.

Matrix Effects on Electrochemical Oxidation of Per- and Polyfluoroalkyl Substances in Sludge Centrate

This study investigated the electrochemical oxidation of per- and polyfluoroalkyl substances (PFAS) using a Ti4O7 anode in centrate from sludge dewatering. Synthetic solutions containing perfluorooctanoic acid (PFOA), other PFAS, and inorganic constituents (phosphate, ammonium, chloride, carbonate, and acetate salts) found in centrate were studied to assess their impact on the oxidation process. PFOA removal decreased from 95% in a stable electrolyte (NaClO4) to 81% in a Na2HPO4 electrolyte and 30% in a solution mimicking concentrated centrate. X-ray photoelectron spectroscopy detected phosphate and nitrogen species on the electrode surface. At potentials required to oxidize PFAS (>3.0 V/SHE), phosphate and ammonium were oxidized to radicals that blocked electrode sites, inhibiting PFAS removal and shifting PFOA oxidation from first-order kinetics. The kinetics were accurately modeled using a Langmuir-Hinshelwood approach with a transient inhibition term. Results suggested that phosphate, ammonium, and bicarbonate ions reduced hydroxyl radical availability, thereby limiting PFOA defluorination. In concentrated centrate, 95% of the chemical oxygen demand and 93% of total PFAS were removed after 233 s of electrolysis at 30 mA cm-2. However, partial degradation of perfluorohexanoic acid and accumulation of perfluoroheptanoic acid, attributed to inorganic electrode fouling, suggested the need for a multistage reactor system for more complete PFAS mineralization.

Recent advances in synthesis and application of Magnéli phase titanium oxides for energy storage and environmental remediation

High-temperature reduction of TiO2 causes the gradual formation of structural defects, leading to oxygen vacancy planar defects and giving rise to Magnéli phases, which are substoichiometric titanium oxides that follow the formula Ti n O2n-1, with 4 ≤ n ≤ 9. A high concentration of defects provides several possible configurations for Ti4+ and Ti3+ within the crystal, with the variation in charge ordered states changing the electronic structure of the material. The changes in crystal and electronic structures of Magnéli phases introduce unique properties absent in TiO2, facilitating their diverse applications. Their exceptional electrical conductivity, stability in harsh chemical environments and capability to generate hydroxyl radicals make them highly valuable in electrochemical applications. Additionally, their high specific capacity and corrosion resistance make them ideal for energy storage facilities. These properties, combined with excellent solar light absorption, have led to their widespread use in electrochemical, photochemical, photothermal, catalytic and energy storage applications. To provide a complete overview of the formation, properties, and environmental- and energy-related applications of Magnéli phase titanium suboxides, this review initially highlights the crystal structure and the physical, thermoelectrical and optical properties of these materials. The conventional and novel strategies developed to synthesise these materials are then discussed, along with potential approaches to overcome challenges associated with current issues and future low-energy fabrication methods. Finally, we provide a comprehensive overview of their applications across various fields, including environmental remediation, energy storage, and thermoelectric and optoelectronic technologies. We also discuss promising new directions for the use of Magnéli phase titanium suboxides and solutions to challenges in energy and environment-related applications, and provide guidance on how these materials can be developed and utilised to meet diverse research application needs. By making use of control measures to mitigate the potential hazards associated with their nanoparticles, Magnéli phases can be considered as versatile materials with potential for next generation energy needs.

The impact of anions on electrooxidation of perfluoroalkyl acids by porous Magnéli phase titanium suboxide anodes

Previous studies have indicated the great performance of electrooxidation (EO) to mineralize per- and polyfluoroalkyl substances (PFASs) in water, but different anions presented in wastewater may affect the implementation of EO treatment in field applications. This study invetigated EO treatment of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), two representative perfluoroalkyl acids (PFAAs), using porous Magnéli phase titanium suboxide anodes in electrolyte solutions with different anions present, including NO3-, SO42-, CO32- and PO43-. The experiment results indicate that CO32- enhanced PFAS degradation, while NO3- suppressed the degradation reactions with its concentration higher than 10 mM. SO42- and PO43- exhibited less impact. Further studies with electrochemical characterizations and radical quenching experiments illustrate the mechanisms of how the anions may impact EO performance.

High photothermal conversion efficiency of RF sputtered Ti4O7 Magneli phase thin films and its linear correlation with light absorption capacity

RF-sputtering is used to deposit Ti4O7-Magneli-phase films onto various substrates at deposition temperatures (Ts) ranging from 25 to 650 °C. Not only the structural, but also electrical conductivity, optical absorbance and photothermal properties of the Ti4O7 films are shown to change significantly with Ts. A Ts of 500 °C is pointed out as the optimal temperature that yields highly-crystalized pure-Ti4O7-Magneli phase with a densely-packed morphology and a conductivity as high as 740 S/cm. The Ti4O7 films deposited at Ts = 450-500 °C also exhibited the highest optical absorption over all the broad (200-1500) nm range. The absorbed sunlight (AM1.5) was efficiently converted into heat by raising the temperature of the Ti4O7 films up to ~ 54 °C. Thus, the external photothermal efficiency (ηext) of the Ti4O7 films, was found to be as high as ~ 74%. This is the highest ηext reported so far for sputtered-Ti4O7 coatings (just ~ 450 nm-thick), highlighting their significant potential for photothermal applications such as desalination, deicing and/or smart windows. Finally, the ηext of the Ti4O7 coatings is demonstrated, for the first time, to be linearly correlated to their integrated light absorption coefficient. This fundamental relationship paves the way towards the design and optimization of highly efficient solar-thermal conversion devices.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Efficient electro-oxidation-based degradation of per- and polyfluoroalkyl (PFAS) persistent pollutants by using plasma torch synthesized pure-Magnéli phase-Ti4O7 anodes

Pure Magnéli-phase Ti4O7 were prepared by means of a Plasma Torch (PT) coating method and integrated into an advanced electro-catalytic oxidation (AEO) process in order to degrade perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) persistent pollutants present in waters. The X-ray diffraction analysis confirmed the polycrystalline nature of the pure Magnéli phase PT-Ti4O7 coatings (∼100 μm thick)). The Raman spectra of the PT-Ti4O7 coatings also exhibited the two characteristic peaks (at 138 and 183 cm-1) of the PT-Ti4O7 Magnéli phase. Scanning electron microscopy revealed the nanostructured hierarchical morphology of the PT-Ti4O7 thus conferring them high surface area. The PT-Ti4O7 anodes are shown to achieve higher degradation efficiencies towards PFOA and PFOS in comparison with the conventional boron-doped diamond anodes. By investigating several AEO parameters (including current density, treatment time, nature of the anode material), we were able to optimise the AEO process. Thus, for both PFOA and PFOS (at an initial concentration of 500 ppb in synthetic wastewaters), degradation efficiencies as high as 96.6% and 99.7% were achieved, respectively, with a current density of 20 mA/cm2, a treatment time of 120 min and PT-Ti4O7 mesh-type anodes. PFOA and PFOS can be degraded by both direct anodic electrochemical oxidation (•OH radicals) and indirect electrochemical oxidation via mediators, such as persulphate acid (H2S2O8) generated by sulphate anodic oxidation. The degradation of both compounds followed pseudo-first-order kinetics. The reaction rate constant (k) for PFOS removal was 4.63 × 10-2 min-1, whereas 2.76 × 10-2 min-1 was recorded for PFOA removal. Subsequently, we have used the above optimal AEO operating conditions to treat real wastewater effluents (containing 17 types of PFAS molecules with a total content of 8500 ppb) and achieved a degradation rate of 39.1%-87.4% for eight of the 17 PFAS compounds. The degradation rate was found to be dependent on the chemical structure and chain length of each PFOA/PFOS component.

Preparation and Electrochemical Applications of Magnéli Phase Titanium Suboxides: A Review

Magnéli phase titanium suboxides (M-TSOs) belong to a type of sub-stoichiometric titanium oxides based on the crystal structure of rutile TiO2. They possess a unique shear structure, granting them exceptional electrical conductivity and corrosion resistance. These two advantages are crucial for electrode materials in electrochemistry, hence the significant interest from numerous researchers. However, the preparation of M-TSOs is uneconomic due to high temperature reduction and other complex synthesis process, thus limiting their practical application in electrochemical fields. This review delves into the crystal structure, properties, and synthesis methods of M-TSOs, and touches on their applications as electrocatalysts in wastewater treatment and electrochemical water splitting. Furthermore, it highlights the research challenges and potential future research directions in M-TSOs.

In situ studies on defect formation dynamics in flash-sintered TiO2

Flash-sintered (FS) ceramics have shown promising mechanical deformability at room temperature compared to conventional sintered ceramics. One major contributing factor to plasticity is high-density defects, such as dislocations, stacking faults and point defects, resulted presumably from the high electrical field during flash sintering. However, such direct experiemtnal evidence for defect formation and evolution under the electric field remains lacking. Here we performed in situ biasing experiments in FS and conventionally sintered (CS) polycrystalline TiO2 in a transmission electron microscope (TEM) to compare the defect evolution dynamics. In situ TEM studies revealed the coalescence of point defects under the electrical field in both FS and CS TiO2 and the subsequent formation of stacking faults, which are often referred to as Wadsley defects. Surprisingly, under the electrical field, the average fault growth rate in the FS samples is 10 times as much as that in the CS TiO2. Furthermore, the Magnéli phase, a 3D oxygen-deficient phase formed by the aggregation of Wadsley defects, is observed in the FS samples, but not in the CS samples. The present study provides new insights into defect dynamics in FS ceramics.

A Review: Synthesis and Applications of Titanium Sub-Oxides

Magnéli phase titanium oxides, also called titanium sub-oxides (TinO2n-1, 4 < n < 9), are a series of electrically conducting ceramic materials. The synthesis and applications of these materials have recently attracted tremendous attention because of their applications in a number of existing and emerging areas. Titanium sub-oxides are generally synthesized through the reduction of titanium dioxide using hydrogen, carbon, metals or metal hydrides as reduction agents. More recently, the synthesis of nanostructured titanium sub-oxides has been making progress through optimizing thermal reduction processes or using new titanium-containing precursors. Titanium sub-oxides have attractive properties such as electrical conductivity, corrosion resistance and optical properties. Titanium sub-oxides have played important roles in a number of areas such as conducting materials, fuel cells and organic degradation. Titanium sub-oxides also show promising applications in batteries, solar energy, coatings and electronic and optoelectronic devices. Titanium sub-oxides are expected to become more important materials in the future. In this review, the recent progress in the synthesis methods and applications of titanium sub-oxides in the existing and emerging areas are reviewed.

Ti/Ti4O7 Anodes for Efficient Electrodeposition of Manganese Metal and Anode Slime Generation Reduction

Preventing lead-based anodes from causing high-energy consumption, lead pollution, and harmful anode slime emission is a major challenge for the current electrolytic manganese metal industry. In this work, a Ti4O7-coated titanium electrode was used as anode material (Ti/Ti4O7 anode) in manganese electrowinning process for the first time and compared with a lead-based anode (Pb anode). The Ti/Ti4O7 anode was used for galvanostatic electrolysis; the cathodic current efficiency improved by 3.22% and energy consumption decreased by 7.82%. During 8 h of electrolysis, it reduced 90.42% solution anode slime and 72.80% plate anode slime formation. Anode product characterization and electrochemical tests indicated that the Ti/Ti4O7 anode possesses good oxygen evolution activity, and γ-MnO2 has a positive catalytic effect on oxygen evolution reaction (OER), which inhibited anode Mn2+ oxidation reaction and reduced the formation of anode slime. In addition, the low charge-transfer resistance, high diffusion resistance, and dense MnO2 layer of the anode blocked the diffusion path of Mn3+ in the system and inhibited the formation of anode slime. The Ti/Ti4O7 anode exhibits excellent electrochemical performance, which provides a new idea for the selection of novel anodes, energy savings and emission reduction, and the establishment of a new mode of clean production in the electrolytic manganese metal industry.

Mesoporous Ti4O7 Spheres with Enhanced Zinc-Anchoring Effect for High-Performance Zinc-Nickel Batteries

Zinc-nickel batteries are promising competitors for next-generation power supply due to their benefits of high safety, high working voltage, and attractive rate performance. However, their practical applications are plagued by their poor cycling performance, stemming from uneven redistribution of zinc during cycling that results in dendrite formation and shape changes of the electrode. In this work, mesoporous Ti₄O₇ microspheres are prepared and are employed as additives of a zinc anode. Notably, the presence of mesopores provides abundant chemisorption sites for Zn(OH)₄^2- ions, inhibiting severe zinc redistribution in the electrode. Moreover, due to the good electrical conductivity and mesopores that serve as ion diffusion channels, the reaction reactivity and reversibility of the zinc electrode are greatly facilitated. As a result, the fabricated zinc-nickel battery with mesoporous Ti₄O₇ additives (ms-Ti₄O₇) exhibits an enhanced discharge capacity and a significantly prolonged cycling life. Even at a current of 10 A (∼138 mA cm^-2), the ms-Ti₄O₇-modified anode demonstrates stable operation for longer than 718 h (700 cycles) with a discharge voltage of 1.2 V, which is much longer than those of a ZnO anode (192 h, 117 cycles) and a Ti₄O₇-particle (p-Ti₄O₇)-modified battery (590 h, 443 cycles). Furthermore, due to the anchoring effect for Zn(OH)₄^2- and the uniform electric field, the effect of mesoporous Ti₄O₇ on inhibiting dendrite formation and shape change of the zinc electrode is highlighted.

A comprehensive review on green perspectives of electrocoagulation integrated with advanced processes for effective pollutants removal from water environment

The rapidly expanding global energy demand is forcing a release of regulated pollutants into water that is threatening human health. Among various wastewater remediating processes, electrocoagulation (EC) has scored a monumental success over conventional processes because it combines coagulation, sedimentation, floatation and electrochemical oxidation processes that can effectively decimate numerous stubborn pollutants. The EC processes have gained some attention through various academic and industrial publications, however critical evaluation of EC processes, choices of EC processes for various pollutants, process parameters, mechanisms, commercial EC technologies and performance enhancement via other degradation processes (DPs) integration have not been comprehensively covered to date. Therefore, the major objective of this paper is to provide a comprehensive review of 20 years of literature covering EC fundamentals, key process factors for a reactor design, process implementation, current challenges and performance enhancement by coupling EC with pivotal pollutant DPs including, electro/photo-Fenton (E/P-F), photocatalysis, sono-chemical treatment, ozonation, indirect electrochemical/advanced oxidation (AO), and biosorption that have substantially reduced metals, pathogens, toxic compound BOD, COD, colors in wastewater. The results suggest that the optimum treatment time, current density, pulse frequency, shaking speed and spaced electrode improve the pollutants removal efficiency. An elegant process design can prevent electrode passivation which is a critical limitation of EC technology. EC coupling (up or downstream) with other DPs has resulted in the removal of organic pollutants and heavy metals with a 20% improved efficiency by EC-EF, removal of 85.5% suspended solid, 76.2% turbidity, 88.9% BOD, 79.7% COD and 93% color by EC-electroflotation, 100% decolorization by EC-electrochemical-AO, reduction of 78% COD, 81% BOD, 97% color by EC-ozonation and removal of 94% ammonia, 94% BOD, 95% turbidity, >98% phosphorus by aerated EC and peroxicoagulation. The major wastewater purification achievements, future potential and challenges are described to model the future EC integrated systems.

Magnéli phase titanium sub-oxides synthesis, fabrication and its application for environmental remediation: Current status and prospect

Sub-stoichiometric titanium oxide, also called titanium suboxides (TSO), had been a focus of research for many decades with a chemical composition of Ti_nO_2n-1 (n ≥ 1). It has a unique oxygen-deficient crystal structure which provides it an outstanding electrical conductivity and high corrosion resistance similar to ceramic materials. High electrical conductivity and ability to sustain in adverse media make these phases a point of attention for researchers in energy storage and environmental remediation applications. The Magnéli phase-based reactive electroconductive membranes (REM) and electrodes have demonstrated the electrochemical oxidation of pollutants in the water in flow-through and flow by configuration. Additionally, it has also shown its potential for visible light photochemical degradation as well. This review attempts to summarize state of the art in various Magnéli phases materials synthesis routes and their electrochemical and photochemical ability for environmental application. The manuscript introduces the Magnéli phase, its crystal structure, and catalytic properties, followed by the recent development in synthesis methods from diverse titanium sources, notably TiO₂ through thermal reduction. The various fabrication methods for Magnéli phase-base REMs and electrodes have also been summarized. Furthermore, the article discussed the environmental remediations via electrochemical and photochemical advanced oxidation processes. Additionally, the hybrid technology with REMs and electrodes is used to counter membrane biofouling and develop electrochemical sensing devices for the pollutants. The Magnéli phase materials have a bright future for both electrochemical and photochemical advanced oxidation of emerging contaminants in water and wastewater treatment.

Electrooxidation of per- and polyfluoroalkyl substances in chloride-containing water on surface-fluorinated Ti4O7 anodes: Mitigation and elimination of chlorate and perchlorate formation

Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFASs, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti₄O₇ anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti₄O₇ anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveal the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti₄O₇ anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti₄O₇ anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.

Cost and efficiency perspectives of ceramic membranes for water treatment

More robust ceramic membranes with tailorable structures and functions are increasingly employed for water treatment, particularly in some harsh applications for their ultra-long service lifespan due to their high mechanical, structural, chemical and thermal stability and anti-fouling properties. Decreasing cost and enhancing efficiency are two key but quite challenging application-oriented issues for broader and larger-scale engineering application of current ceramic membranes, and are required to make ceramic membranes a highly efficient and economic water treatment technique. In this review, we critically discuss these two significant concerns of both cost and efficiency for water treatment ceramic membranes, focusing on an overview of various advanced strategies and mechanism insights. A brief up-to-date discussion is first introduced about recent developments of ceramic membranes covering the major advances of novel membranes and applications. Then some promising strategies for decreasing the cost of ceramic membranes are discussed, including membrane material cost and processing cost. To fully address the issue of moderate efficiency with single separation function, valuable and considerable insights are provided into recent major progress and mechanism understandings in application with other unit processes, such as advanced oxidation and electrochemistry techniques, to significantly enhance treatment efficiency. Subsequently, a review of recent ceramic membrane applications emphasizing harsh operating environments is presented, such as oil-water separation, saline water, refractory organic and emerging contaminant wastewater treatment. Finally, engineering application, conclusions, and future perspectives of ceramic membrane for water treatment applications are critically discussed offering new insight based on understanding the issues of cost and efficiency.

Progress in Preparation and Application of Titanium Sub-Oxides Electrode in Electrocatalytic Degradation for Wastewater Treatment

To achieve low-carbon and sustainable development it is imperative to explore water treatment technologies in a carbon-neutral model. Because of its advantages of high efficiency, low consumption, and no secondary pollution, electrocatalytic oxidation technology has attracted increasing attention in tackling the challenges of organic wastewater treatment. The performance of an electrocatalytic oxidation system depends mainly on the properties of electrodes materials. Compared with the instability of graphite electrodes, the high expenditure of noble metal electrodes and boron-doped diamond electrodes, and the hidden dangers of titanium-based metal oxide electrodes, a titanium sub-oxide material has been characterized as an ideal choice of anode material due to its unique crystal and electronic structure, including high conductivity, decent catalytic activity, intense physical and chemical stability, corrosion resistance, low cost, and long service life, etc. This paper systematically reviews the electrode preparation technology of Magnéli phase titanium sub-oxide and its research progress in the electrochemical advanced oxidation treatment of organic wastewater in recent years, with technical difficulties highlighted. Future research directions are further proposed in process optimization, material modification, and application expansion. It is worth noting that Magnéli phase titanium sub-oxides have played very important roles in organic degradation. There is no doubt that titanium sub-oxides will become indispensable materials in the future.

Gas Sensors Based on Titanium Oxides (Review)

Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO2) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying significant attention to the variety of allotropic modifications of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO2−x)-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO2 and conducting polymer-based composite materials were developed, which have found some specific application areas. Therefore, in this review, we are highlighting how specific formation methods, which can be used for the formation of TiO2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these specific technological fields. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined.

The effects of dietary exposure to Magnéli phase titanium suboxide and titanium dioxide on rainbow trout (Oncorhynchus mykiss)

Magnéli phase titanium suboxides (Magnéli TiO_x) are promising, novel materials with superior properties compared to TiO₂, they are substoichiometric titanium oxides with the chemical formula Ti_nO_2n-1 (where n ≥ 1). In this study, for the first time, subchronic effects of dietary intake of Magnéli TiO_x were evaluated and compared with TiO₂ particles of similar size, in concentrations 0.1% and 0.01% of feed. The experiment consisted of 38 d of an exposition period and 14 d of a depuration period. Minor effects on plasma biochemical profile and morphological parameters were recorded. A reduced count of leukocytes was found in the blood of both Magnéli TiO_x and TiO₂ exposed fish, suggesting immunotoxic effects. Erythrocytosis was specific for Magnéli TiO_x. Indices of oxidative stress, namely increased lipid peroxidation in liver, increased activity of superoxide dismutase in liver, kidney and gills and glutathione S-transferase (GST) in gills, as well as decreased activity of ceruloplasmin and GST in liver were found predominantly in fish exposed to TiO₂. Histopathological examination revealed increased lipid-like vacuolation in the liver, the presence of hyaline droplets in renal tubules and multiplication of mucous glands in the epidermis in both tested substances and intestine damage in TiO₂ groups. Overall, in Magnéli TiO_x exposed groups, fewer adverse effects compared to TiO₂ expositions were recorded. Their wider practical implementation in place of TiO₂ is therefore beneficial.

Atomic layer deposited Ti2O3 thin films

Ti2O3 thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti2O3 thin films, the insulator-metal transition is observed at ~80 K, with nearly 3-4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depends on the crystallographic c/a ratio leads to a change in electrical resistivity against temperature.

Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

TD-DFT calculations were performed on neutral Ti_nO_2n, Ti_nO_2n-1, and Ti_nO_2n-2 clusters, where n ≤ 7. Calculations show the Ti_nO_2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the suboxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcitations of Ti_nO_2n clusters are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters are all characterized by d-d transitions that occur at a much smaller optical gap. Upon low energy photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized around the cluster. The properties of the clusters, including the oxygen binding energies and structures, were calculated to account for the variation in relative populations found in experimental cluster distributions. Several Ti_nO_2n-2 clusters contain higher symmetry which is reflected in their relative stability. In particular, the tetrahedral symmetry of Ti₄O₆ inhibits charge carrier localization and therefore exhibits higher stability.

Inexpensive gram scale synthesis of porous Ti4O7 for high performance polymer electrolyte fuel cell electrodes

As a fuel cell catalyst support, more than 2 g of Magnéli phase Ti₄O₇ fine-particles were synthesized in a single reaction via an inexpensive route. The single-cell performance reached that of commercial carbon-supported platinum, with an excellent load cycle durability, one of the highest ever reported for oxide-supported platinum catalysts.

Stacked Two-Dimensional MXene Composites for an Energy-Efficient Memory and Digital Comparator

Two-dimensional MXene has enormous potential for application in industry and academia owing to its surface hydrophilicity and excellent electrochemical properties. However, the application of MXene in optoelectronic memory and logical computing is still facing challenges. In this study, an optoelectronic resistive random access memory (RRAM) based on silver nanoparticles (Ag NPs)@MXene-TiO₂ nanosheets (AMT) was prepared through a low-cost and facile hydrothermal oxidation process. The fabricated device exhibited a typical bipolar switching behavior and controllable SET voltage. Furthermore, we successfully demonstrated a 4-bit in-memory digital comparator with AMT RRAMs, which can replace five logic gates in a traditional approach. The AMT-based digital comparator may open the door for future integrated functions and applications in optoelectronic data storage and simplify the complex logic operations.

Carbon-Based Composites as Anodes for Microbial Fuel Cells: Recent Advances and Challenges

Owing to the low price, chemical stability and good conductivity, carbon-based materials have been extensively applied as the anode in microbial fuel cells (MFCs). In this review, apart from the charge storage mechanism and anode requirements, the major work focuses on five categories of carbon-based anode materials (traditional carbon, porous carbon, nano-carbon, metal/carbon composite and polymer/carbon composite). The relationship is demonstrated in depth between the physicochemical properties of the anode surface/interface/bulk (porosity, surface area, hydrophilicity, partical size, charge, roughness, etc.) and the bioelectrochemical performances (electron transfer, electrolyte diffusion, capacitance, toxicity, start-up time, current, power density, voltage, etc.). An outlook for future work is also proposed.

A tubular electrode assembly reactor for enhanced electrochemical wastewater treatment with a Magnéli-phase titanium suboxide (M-TiSO) anode and in situ utilization

The electrochemical oxidation technology has been widely used for the waste water treatment and water reuse because of its easy-to-operate nature, an effective removal of pollutants and non-secondary pollution. However, the price of electrode materials, the limitation of mass transfer and the associated effects on contaminant degradation hamper its application. Within this context, an in situ utilization tubular electrode assembly reactor (TEAR) was proposed, in which a stainless steel pipe (SSP) was used as the cathode, and a tubular Magnéli-phase titanium suboxide (M-TiSO) anode was posited in the center of that pipe. Besides the cathode and anode, an integral electrochemical system to treat water pollutants was constituted with a spiral static mixer made from three-dimensional (3D) printing. A spiral static mixer was pushed into the interspace of electrodes to minimize the adverse effect caused by inhomogeneous distribution of pollutants. Here, the effects of current density and resident time on the removal of methylene blue (MB) and total organic carbon (TOC) were investigated, the corresponding hydrodynamics was studied using computational fluid dynamics (CFD), and the long-term stability of removing MB by the reactor was discussed. The results indicated that the MB and TOC removal rate was enhanced at specific current density with a static mixer and the velocity distribution tended to be more homogeneous. Moreover, the anode surface shear force and heat transfer were increased by improving the fluid state. This study proposed an in situ utilization concept and provided a potential value for feasible and efficient water treatment.

A stainless steel pipe (SSP) was used as a cathode. A tubular Magnéli-phase titanium suboxide (M-TiSO) anode was posited in the center. A spiral static mixer was used to process intensification.

Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability

Lithium-ion batteries, which have revolutionized portable electronics over the past three decades, were eventually recognized with the 2019 Nobel Prize in chemistry. As the energy density of current lithium-ion batteries is approaching its limit, developing new battery technologies beyond lithium-ion chemistry is significant for next-generation high energy storage. Lithium-sulfur (Li-S) batteries, which rely on the reversible redox reactions between lithium and sulfur, appears to be a promising energy storage system to take over from the conventional lithium-ion batteries for next-generation energy storage owing to their overwhelming energy density compared to the existing lithium-ion batteries today. Over the past 60 years, especially the past decade, significant academic and commercial progress has been made on Li-S batteries. From the concept of the sulfur cathode first proposed in the 1960s to the current commercial Li-S batteries used in unmanned aircraft, the story of Li-S batteries is full of breakthroughs and back tracing steps. Herein, the development and advancement of Li-S batteries in terms of sulfur-based composite cathode design, separator modification, binder improvement, electrolyte optimization, and lithium metal protection is summarized. An outlook on the future directions and prospects for Li-S batteries is also offered.

Combining Zirconia and Titanium Suboxides by Vat Photopolymerization

A recently developed multi-ceramic additive manufacturing process (multi-CAMP) and an appropriate device offer a multi-material approach by vat photopolymerization (VPP) of multi-functionalized ceramic components. However, this process is limited to ceramic powders with a certain translucency for visible light. Electrically conductive ceramic powders are therefore ruled out because of their light-absorbing behavior and dark color. The goal of the collaborative work described in the article was to develop a material combination for this multi-material approach of the additive vat photopolymerization method which allows for combining electrical conductivity and electrical insulation plus high mechanical strength in co-sintered ceramic components. As conductive component titanium suboxides are chosen, whereas zirconia forms the mechanically stable and insulation part. Since titanium suboxides cannot be used for vat photopolymerization due to their light-absorbing behavior, titania is used instead. After additive manufacturing, the two-component parts are co-sintered in a reducing atmosphere to transform the titania into its suboxides and, thus, attaining the desired property combination. The article describes the challenges of the co-processing of both materials due to the complex optical properties of titania. Furthermore, the article shows successfully co-sintered testing parts of the material combination of zirconia/titanium suboxide which are made by assembling single-material VPP components in the green state and subsequent common thermal treatment. The results of microstructural and interface investigations such as electrical measurements are discussed.

Progress and Insights in the Application of MXenes as New 2D Nano-Materials Suitable for Biosensors and Biofuel Cell Design

Recent progress in the application of new 2D-materials-MXenes-in the design of biosensors, biofuel cells and bioelectronics is overviewed and some advances in this area are foreseen. Recent developments in the formation of a relatively new class of 2D metallically conducting MXenes opens a new avenue for the design of conducting composites with metallic conductivity and advanced sensing properties. Advantageous properties of MXenes suitable for biosensing applications are discussed. Frontiers and new insights in the area of application of MXenes in sensorics, biosensorics and in the design of some wearable electronic devices are outlined. Some disadvantages and challenges in the application of MXene based structures are critically discussed.

Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and VOCs (TiO2-x and TinO2n-1 vs. TiO2)

In this review article, attention is paid towards the formation of various nanostructured stoichiometric titanium dioxide (TiO2), non-stoichiometric titanium oxide (TiO2-x) and Magnéli phase (TinO2n-1)-based layers, which are suitable for the application in gas and volatile organic compound (VOC) sensors. Some aspects related to variation of sensitivity and selectivity of titanium oxide-based sensors are critically overviewed and discussed. The most promising titanium oxide-based hetero- and nano-structures are outlined. Recent research and many recently available reviews on TiO2-based sensors and some TiO2 synthesis methods are discussed. Some promising directions for the development of TiO2-based sensors, especially those that are capable to operate at relatively low temperatures, are outlined. The applicability of non-stoichiometric titanium oxides in the development of gas and VOC sensors is foreseen and transitions between various titanium oxide states are discussed. The presence of non-stoichiometric titanium oxide and Magnéli phase (TinO2n-1)-based layers in 'self-heating' sensors is predicted, and the advantages and limitations of 'self-heating' gas and VOC sensors, based on TiO2 and TiO2-x/TiO2 heterostructures, are discussed.

Titanium Monoxide-Stabilized Silicon Nanoparticles with a Litchi-like Structure as an Advanced Anode for Li-ion Batteries

Silicon (Si) has been considered as the most potential anode material for next-generation high-energy density lithium-ion batteries (LIBs) because of its extremely high theoretical capacity. However, the performance deterioration caused by volume change and low electrical conductivity of active Si particles greatly limit its commercial use. Here, we designed a nonstoichiometric TiO_x-coated Si anode with a litchi-like structure, in which Si-Ti and Si-O dual bonds are expected to form between the Si core and TiO_x shell. This unique structure plays a major role in preventing the volume expansion and improving the electrical conductivity of the Si anode. The as-prepared TiO_x-coated Si anode could exhibit excellent cycling stability after 1000 cycles at 1000 mA g^-1 with a relatively small capacity decay rate of ∼0.04% per cycle, which can be comparable to most of the modified Si anodes in references. This strategy of surface regulating on the Si anode could be extended to other electrodes with large volume expansion during cycling in LIBs for achieving competitive electrochemical properties.

Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals

Electro-generated hydroxyl radicals (•OH) are of fundamental importance to the electrochemical advanced oxidation process (EAOP). Radical-specific electron spin resonance (ESR) evidence is still lacking in association with the direct electron transfer (DET) reaction of spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide; DMPO) and side reactions of the DMPO-OH adduct in the strongly oxidative environment offered by anodic polarization. Herein, we showed ESR identification of electro-generated •OH in EAOP based on the principle of kinetic selection. Excessive addition of a DMPO agent and fast spin trapping allowed suitable kinetic conditions to be set for effective spin trapping of electro-generated •OH and subsequent ESR identification. Otherwise, interferential triplet signals would emerge due to formation of paramagnetic dimer via dehydrogenation, DET oxidation, and dimerization reactions of the DMPO-OH adduct. The results demonstrate that •OH formation during spin-trapping on the titanium suboxide (TiSO) anode could be quantified as 47.84 ± 0.44 μM at current density of 10 mA cm^-2. This value revealed a positive dependence on electrolysis time, current density, and anode potential. The effectiveness of ESR measurements was verified by the results obtained with the terephthalic acid probe. The ESR identification not only provides direct evidence for electro-generated •OH from a fundamental point of view, but also suggests a strategy to screen effective anode materials.

Amorphous Pd-Loaded Ti4O7 Electrode for Direct Anodic Destruction of Perfluorooctanoic Acid

We here present a novel Ti₄O₇-based electrode loaded with amorphous Pd clusters that achieve efficient anodic destruction of perfluorooctanoic acid (PFOA), a persistent water pollutant with significant environmental and human health concerns. These amorphous Pd clusters were characterized by the disordered, noncrystalline arrangement of Pd single atoms in close proximity, in contrast to crystalline Pd nanoparticles that have been often employed to tailor the electronic properties of an electrode. We found that the Ti₄O₇ electrode loaded with amorphous Pd clusters significantly outperformed the Ti₄O₇ electrode loaded with crystalline Pd particles due to enhanced electron transfer through dominant Pd-O bonds. Combined with the efficient binding of PFOA and its degradation intermediates to the fluorinated electrode surface, this electrode was capable of mineralizing PFOA and releasing fluoride as F^-. The reaction pathway was found to proceed without involving reactive oxygen species and therefore was not quenched by common anions in complex natural water systems such as chloride ions.

Electrochemical oxidation of methyl orange by a Magnéli phase Ti4O7 anode

In this study, high quality Magnéli phase Ti₄O₇ bulks with electrical conductivity up to 961.5 S cm^-1 were successfully prepared by spark plasma sintering (SPS) and then served as electrode materials for electrochemical oxidation of azo dye methyl orange (MO). The influences of current density and initial dye concentration on the removal rates of MO and chemical oxygen demand (COD) were studied. Removal of MO and COD exhibited an increase with increasing current density and decreasing initial concentration of MO. Complete removal of MO was realized within a short time under all experimental conditions. The removal rate of COD reached 91.7% when current density was 10 mA cm^-2 and initial dye concentration was 100 mg L^-1. In addition, the electrochemical oxidation rate could be described through a pseudo-first-order kinetic constant k, and the obtained experimental results could be well fitted with a proposed kinetic model in all the examined conditions. Possible degradation mechanisms for electrochemical oxidation of MO by Ti₄O₇ electrode were proposed on the basis of intermediate products analysis. Tests were also conducted with other commercial electrodes for comparison, including commercial graphite, stainless-steel and dimension stable anode (DSA) electrodes. The results showed that Ti₄O₇ anode exhibited the fastest electrochemical oxidation rates than those of the other electrodes. This study provides a feasible method for realizing high efficiency of electrochemical oxidation degradation by Ti₄O₇ electrode.

Performance evaluation of treating oil-containing restaurant wastewater in microbial fuel cell using in situ graphene/polyaniline modified titanium oxide anode

Most studies conducted nowadays to boost electrode performance in microbial fuel cell (MFC) have focused on carbonaceous materials. The titanium suboxides (Ti₄O₇, TS) are able to provide a new alternative for achieving better performance in MFC and have been tested and demonstrated in this study. The Ti₄O₇ electrode with high electrochemical activity was modified by graphene/polyaniline by the constant potential method. Electrogenic microorganisms were more conducive to adhere to the anode electrode due to the presence of graphene/polyaniline. The MFC reactor with polyaniline /graphene modified TS (TSGP) anode achieves the highest voltage with 980 mV, and produces a peak power density of 2073 mW/m², which is 2.9 and 12.7 times of those with the carbon cloth anode, respectively, at the 1000 Ω external resistance. In addition, this study evaluates the effects of anolyte conductivity, pH, and COD on the treatment of oil-containing restaurant wastewater (OCRW) in MFC using TSGP anode. The OCRW amended with 120 mS/cm obtains the lowest internal resistance (160.3 Ω). Increasing the anodic pH, gradually from acidic (pH 5.5) to alkaline conditions (pH 8.0), resulted in a gradual increase in maximum power density to 576.4 mW/m² and a decrease in internal cell resistance to 203.7 Ω. The MFC at the COD 1500 mg/L could obtain steady-state output voltage during 103 h while removing up to 65.2% of the COD of the OCRW.

TiO2-x/TiO2-Structure Based 'Self-Heated' Sensor for the Determination of Some Reducing Gases

In this research we report the gas-sensing properties of TiO2-x/TiO2-based hetero-structure, which was 'self-heated' by current that at constant potential passed through the structure. Amperometric measurements were applied for the evaluation of sensor response towards ethanol, methanol, n-propanol and acetone gases/vapours. The sensitivity towards these gases was based on electrical resistance changes, which were determined by amperometric measurements of current at fixed voltage applied between Pt-based contacts/electrodes deposited on the TiO2-x/TiO2-based layer. X-ray diffraction (XRD) analysis revealed the formation of TiO2-x/TiO2-based hetero-structure, which is mainly based on Ti3O5/TiO2 formed during the hydro-thermal oxidation-based sensing-layer preparation process. Additionally, photoluminescence and time-resolved photoluminescence decay kinetics-based signals of this sensing structure revealed the presence of TiO2 mainly in the anatase phase in the TiO2-x/TiO2-based hetero-structure, which was formed at 400 °C annealing temperature. The evaluation of TiO2-x/TiO2-based gas-sensing layer was performed at several different temperatures (25 °C, 72 °C, 150 °C, 180 °C) and at these temperatures different sensitivity to the aforementioned gaseous materials was determined.

Size Control of Ti4O7 Nanoparticles by Carbothermal Reduction Using a Multimode Microwave Furnace

Utilization of Ti4O7 in applications such as catalyst support calls for control over the size of the Ti4O7 nanoparticles. This can be achieved using a simple process such as carbothermal reduction. In this study, various sizes of Ti4O7 nanoparticles (25, 60, and 125 nm) were synthesized by carbothermal reduction using a multimode microwave apparatus. It was possible to produce Ti4O7 nanoparticles as small as 25 nm by precisely controlling the temperature, heating process, and holding time of the sample while taking advantage of the characteristics of microwave heating such as rapid and volumetric heating. The results show that microwave carbothermal reduction is advantageous in controlling the size of the Ti4O7 nanoparticles.

Electrochemical inactivation of bacteria with a titanium sub-oxide reactive membrane

A reactive electrochemical membrane (REM) system was developed with titanium suboxide microfiltration membrane serving as the filter and the anode, and was examined to inactivate Escherichia coli (E. coli) in water at various current densities. After passing through the membrane filter, the concentration of E. coli decreased from 6.46 log CFU/mL to 0.18 log CFU/mL. The REM operation and effects, including membrane pressure, anode potential, protein leakage, and cell morphology, were characterized under different treatment conditions. It was found that several mechanisms, including membrane filtration, external electrical field influence, and direct oxidation, functioned in concert to lead to bacteria removal and inactivation, and direct oxidation likely played the major role. As revealed by scanning electron microscope and extracellular protein analysis, high current density and voltage caused severe cell damage that resulted in partial or complete cell disintegration. The removal of a model virus, bacteriophage MS2, was also investigated at the current density of 10 mA cm^-2 and achieved 6.74 log reduction compared to the original concentration (10¹¹ PFU/mL). In addition to illustration of mechanisms, this study may provide a potentially promising approach that is suitable for decentralized treatment to meet dispersed water disinfection needs.

Electro-oxidation of organic pollutants by reactive electrochemical membranes

Electro-oxidation processes are promising options for the removal of organic pollutants from water. The major appeal of these technologies is the possibility to avoid the addition of chemical reagents. However, a major limitation is associated with slow mass transfer that reduces the efficiency and hinders the potential for large-scale application of these technologies. Therefore, improving the reactor configuration is currently one of the most important areas for research and development. The recent development of a reactive electrochemical membrane (REM) as a flow-through electrode has proven to be a breakthrough innovation, leading to both high electrochemically active surface area and convection-enhanced mass transport of pollutants. This review summarizes the current state of the art on REMs for the electro-oxidation of organic compounds by anodic oxidation. Specific focuses on the electroactive surface area, mass transport, reactivity, fouling and stability of REMs are included. Recent advances in the development of sub-stoichiometric titanium oxide REMs as anodes have been made. These electrodes possess high electrical conductivity, reactivity (generation of ^•OH), chemical/electrochemical stability, and suitable pore structure that allows for efficient mass transport. Further development of REMs strongly relies on the development of materials with suitable physico-chemical characteristics that produce electrodes with efficient mass transport properties, high electroactive surface area, high reactivity and long-term stability.

Electrically controlled transformation of memristive titanates into mesoporous titanium oxides via incongruent sublimation

Perovskites such as SrTiO3, BaTiO3, and CaTiO3 have become key materials for future energy-efficient memristive data storage and logic applications due to their ability to switch their resistance reversibly upon application of an external voltage. This resistance switching effect is based on the evolution of nanoscale conducting filaments with different stoichiometry and structure than the original oxide. In order to design and optimize memristive devices, a fundamental understanding of the interaction between electrochemical stress, stoichiometry changes and phase transformations is needed. Here, we follow the approach of investigating these effects in a macroscopic model system. We show that by applying a DC voltage under reducing conditions on a perovskite slab it is possible to induce stoichiometry polarization allowing for a controlled decomposition related to incongruent sublimation of the alkaline earth metal starting in the surface region. This way, self-formed mesoporous layers can be generated which are fully depleted by Sr (or Ba, Ca) but consist of titanium oxides including TiO and Ti3O with tens of micrometre thickness. This illustrates that phase transformations can be induced easily by electrochemical driving forces.